In general, a popularized amplification type solid-state imaging device has a pixel part having an amplification function and a scanning circuit placed in the periphery of the pixel part, where data are read from the pixel part by the scanning circuit.
As one example of the amplification type solid-state imaging device, an APS (Active Pixel Sensor) type image sensor in which the pixel part is constructed of CMOS (complementary metal oxide semiconductor) advantageous for integration of the pixel part with a peripheral driving circuit and a signal processing circuit is known. Among the APS type image sensors, a four-transistor type capable of obtaining a high image quality is becoming mainstream in recent years.
FIG. 8 is a diagram showing the pixel structure of a four-transistor type that has four MOS type transistors in a pixel part 110. In this case, a light-receiving portion 101 is usually constructed of an embedded photodiode, and a signal charge is transferred from the light-receiving portion 101 to a signal charge accumulating portion 103 (FD) by a transfer transistor 102. The signal charge accumulating portion 103 is reset with a power voltage Vd by a reset transistor 104 before the signal charge is transferred from the light-receiving portion 101. Subsequently, the transfer transistor 102 is turned on to transfer the signal charge. A voltage at the signal charge accumulating portion 103 after the resetting and after the signal charge transfer is amplified by an amplifying transistor 105, read to a read signal line 107 via a select transistor 106, received by a constant current load 111 connected to an end terminal and outputted to the subsequent stage.
In the pixel structure, during reset operation, the voltage VFD of the signal charge accumulating portion 103 becomes expressed by the following equation.VFD=Vd−Vth  (1)
In this case, Vd is the drain voltage (power voltage) of the reset transistor 104, and Vth is the threshold voltage of the reset transistor 104. However, a backgating effect is produced because source voltage VFD>0, and the voltage becomes higher than a value Vth(0) when source voltage VFD=0. Therefore, according to Equation (1), the reset voltage generally becomes lower than the drain voltage.
In a case where the voltage of the signal charge accumulating portion 103 after the resetting is low, when the transfer transistor 102 is turned on to transfer the charge from the embedded photodiode 101 to the signal charge accumulating portion 103, a voltage difference between the two becomes insufficient, and complete transfer becomes difficult. That is, an electric charge remains in the photodiode 101, and a residual image occurs.
Accordingly, as a method for raising the voltage of the signal charge accumulating portion in the reset operation, the method of Document 1 (Keiji MABUCHI and seven others, “FD-Driving type CMOS sensor and its low-voltage drive technology” The Institute of Image Information and Television Engineers, Mar. 26, 2004, Vol. 28, No. 23, PP. 35-38) is proposed. FIG. 9 shows the pixel structure of the amplification type solid-state imaging device described in Document 1. FIG. 9 shows a light-receiving portion 201, a transfer transistor 202, a constant current load 211, and a switch transistor 212.
In FIG. 9, the source of the amplifying transistor 205 is connected not via the select transistor but directly to a read signal line 207. By turning on the reset transistor 204 in the reset operation, the voltage of the signal charge accumulating portion 203 rises first to (Vd−Vth) in a short time. Subsequently, the voltage of the read signal line 207 rises comparatively slowly and further rises by a due to a capacitance between the gate and the source of the amplifying transistor 205. That is, the voltage VFD of the signal charge accumulating portion 203 in this case becomes expressed by the following equation.VFD=Vd−Vth+α  (2)
However, the method of Document 1 has the following problem.
In the above Equation (2), the threshold voltage Vth of the reset transistor 204 becomes higher than Vth(0) due to the backgating effect and generally becomes about 0.8 V. On the other hand, the value of α depends on the capacitance between the gate and the source of the amplifying transistor 205 and the capacitance of the signal charge accumulating portion 203, and the value is about 0.4 V in the example of Document 1. Therefore, it is difficult for the value of the voltage VFD of the signal charge accumulating portion 203 to exceed the power voltage Vd.
Accordingly, the object of the invention is to provide an amplification type solid-state imaging device capable of sufficiently securing a voltage difference between a photoelectric transducer and a signal charge accumulating portion when a transfer transistor is turned on by increasing the voltage of the signal charge accumulating portion in the reset operation and easily achieving complete transfer (=no residual image) of a signal charge from the photoelectric transducer to the signal charge accumulating portion.